1. Technical Field
The present invention generally relates to trunk piston engine lubricating oil compositions.
2. Description of the Related Art
Trunk piston engines operate using various types and qualities of diesel fuels and heavy fuel oils. Heavy fuel oils typically contain high concentrations of asphaltenes; generally the heaviest and most polar fraction of petroleum. Asphaltenes are highly complex compounds believed to be composed of polyaromatic sheets containing alkyl side chains, and are generally insoluble in lubricating oils. When heavy fuel oils and conventional lubricant oil compositions mix in different temperature regions of a trunk piston engine, black sludge (such as asphaltene deposits or other deposits) and other asphaltene derived deposits (such as undercrown deposits) tend to form. The formation of black sludge or deposits can adversely affect the service interval and maintenance cost of the trunk piston engine. Therefore, an important performance aspect of these lubricants is residual fuel compatibility.
Salicylates have for many years been found to be effective in reducing deposits in trunk piston engine oils, in particular deposits due to asphaltene deposition from residual fuels. A trunk piston engine is generally a medium speed, 4-cycle engine in which the connecting rod is attached directly to the piston. For many years, commercial salicylates used in trunk piston engine oils were derived from C14 to C18 olefins.
Recently it has been shown that salts of alkyl-substituted hydroxyaromatic carboxylic acids, wherein at least 90% of the alkyl groups are C20 or greater, result in improved detergency in trunk piston engine oils. For example, U.S. Patent Application Publication No. 20090281009 which discloses a lubricating oil composition containing (a) a major amount of a Group I base oil and/or a Group II base oil; and (b) at least one detergent comprising a salt of an alkyl-substituted hydroxybenzoic acid, wherein at least 90% of the alkyl groups are C20 or greater, wherein the lubricating oil composition is a medium or high soap formulation.
U.S. Patent Application Publication No. 20070027043 (“the '043 application”) discloses a process for preparing an overbased alkaline earth metal alkylhydroxybenzoate wherein the alkyl moiety of the alkylhydroxybenzoate is a linear aliphatic group having from about 12 to 40 carbon atoms, a branched aliphatic group having from about 9 to 24 carbon atoms or a mixture of linear and branched aliphatic groups. The '043 application further discloses that the overbased alkaline earth metal alkylhydroxybenzoate is useful as a lubricating oil additive in lubricating oil compositions for the lubrication of mechanical components in land and marine engines such as, for example, hydraulic systems, transmissions, two-stroke and four-stroke vehicular engines, trunk piston and two stroke crosshead marine engines.
U.S. Patent Application Publication No. 20100062957 which discloses a method of reducing asphaltene precipitation (black paint) in an engine by lubricating the engine with a lubricating oil composition comprising, or made by admixing (A) an oil of lubricating viscosity in a major amount; and (B) a salicylate detergent system in a minor amount comprising one or more neutral or overbased alkaline earth metal C22 hydrocarbyl substituted salicylates; with the proviso that the salicylate detergent system does not include an alkali metal salicylate.
Heretofore, there has been no recognition or appreciation that a salt of an alkyl-substituted hydroxyaromatic carboxylic acid, wherein the alkyl moiety of the alkyl-substituted hydroxyaromatic carboxylic acid comprises C26+ alkyl groups derived from one or more normal alpha olefins, significantly and unexpectedly improves heavy fuel oil compatibility in a trunk piston engine lubricating oil composition containing a major amount of an oil of lubricating viscosity and one or more zinc dithiophosphates in an amount of about 0.02 to about 0.12 wt. % in terms of phosphorus content.